In order to provide vacuum for a pneumatic brake booster, the interior of which is subdivided into at least one vacuum chamber and a working chamber, use is made of vacuum pumps which suck residual air out of the vacuum chamber and expel it into the atmosphere. In the automobile industry, vane-cell pumps or swing-vane pumps are normally used for this purpose. As a result of their principle, these exhibit a great deal of friction and have to be lubricated in order to achieve an acceptable service life. Vacuum pumps with vanes driven by the internal combustion engine of the motor vehicle are therefore connected to the oil circuit of the internal combustion engine. Nevertheless, a noticeable proportion of the power output by the internal combustion engine has to be expended to drive such a pump. This is the case even when the vacuum in the chamber to be evacuated has already been built up completely. It is therefore expedient to operate the vacuum pump with electrical energy and to switch it on only when the absolute pressure in the vacuum chamber rises above a predetermined value.
Furthermore, in vehicles with electric or hybrid drive, the vacuum pump cannot be driven by the internal combustion engine or cannot be driven from time to time. Therefore, electrically driven vacuum pumps are used in these vehicles.
Equipping such an electrically driven pump with a lubricant circuit or connecting one to such a circuit would mean a disproportionately high expenditure. Thus, for use in motor vehicles having brake systems with electrically driven vacuum pump, only dry-running vacuum pumps are suitable. In vane-cell pumps, the self-lubricating material graphite is used for this purpose, from which the vanes are produced with the necessary precision with a great deal of effort. Therefore, efforts are directed at using a diaphragm pump for the electrical provision of braking vacuum.
A generic mode-pump unit is known, for example, from DE 10 2007 005 223 A1, which is incorporated by reference. The known motor-pump unit has a diaphragm unit with a working diaphragm which is firmly connected to a tappet. The tappet is in turn fixed to a connecting rod by welding or screwing.